Some bad advice. Some good.
I did this, back in 2004 or so, on a 7x.
1. You dont need lots of torque. 1.5 Nm or so is enough.
2. You do need resolution. Nothing is enough.
3. Your aim is for rigidity, and more mechanical resolution leads to more repeatability and rigidity.
4. You need high acceleration, for pullouts only, in threading. Nothing else.
5. You dont need high speed.
The best combo, minimax solution, is 2M542 type chinese drivers (40), 48V DC PSU, nema 23 steppers of 2.5-4 Nm (30-40), belt drives of HTD5-15, at 1:2 - 1:3 (15 -20 € or so).
On steppers, use 14 mm teeth minimum for good belt wrap (12 is too low, imo, ime).
Your step size will be 2000 at 1:3 for 6000 steps/turn.
Your mechanical resolution is still 1/2 step, or 400 steps/turn.
On typical 1 mm screw => 400 steps/mm, which is just right.
Mach3 threading is broken. It will sort-of-work but cannot be relied on (lots and lots of gotchas and it depends).
The only good sensor for threading is an optical encoder.
You need a fast, crisp, signal, with predictable delay at widely varying speeds from 50 to 600 rpm.
A fussy sensor will create fuzzy quality threads.
Suggest polabs cncaddon motion control solution, ethernet, and mach4.
120€.
They have excellent drivers/support/tech for cheaper stuff.
It can read an encoder z/ pulse, ie has support for fast signals.
It will support encoder threading.
Bigger steppers will have lower max rpm, and will accelerate slower.
If you need to go faster than nema 23, its about the same price as big steppers/drivers, and 10x better, to go with modern, cheap, ac brushless servos.
Background:
I have been working on high quality threading with machx for over 10 years, and 2000 hours.
My current system is ac brushless servos, of 10.000 counts/turn, AC220V, 750W, on x and z.
Its very expensive, and meant for me, to be able to do "perfect" threads (industrially).
500 kHz on X,Z, C axis.
4000 steps/mm, or 0.25 microns step size (=/= resolution).
CSMIO-IP-S, industrial controller.